1. Field of the Invention
This invention relates to a thermoplastic elastomer composition, and a formed article and a composite formed article of the same; more particularly to a thermoplastic elastomer composition excellent in softness, permanent compression set (both short- and long-term) and moldability, and having controlled bleed-out of the softening agent, as well as a formed article and a composite formed article of the same.
2. Description of the Art
Recently, thermoplastic elastomers, which are soft materials with rubber elasticity, need no vulcanization, and are as moldable and recyclable as thermoplastic resins, have been extensively used in various areas, such as in automobile parts, home electric appliances, wire coatings, medical equipment parts, footwear and miscellaneous goods.
Of the various thermoplastic elastomers, polystyrene-based elastomers, e.g., styrene-butadiene block copolymers (SBS) and styrene-isoprene block copolymers (SIS) which are block copolymers of an aromatic vinyl compound and an conjugated diene compound, are highly soft, have good rubber elasticity at normal temperature and give highly moldable thermoplastic elastomer compositions. As such, they are extensively used as substitutes for vulcanized rubber.
The elastomer compositions containing styrene and conjugated diene block copolymer, as one type of the above elastomers, whose intramolecular double bonds are hydrogenated, are more extensively used since they have improved resistance to aging under heat (thermal stability) and to weather.
However, the thermoplastic elastomer compositions containing the hydrogenated block copolymer still involves some problems in terms of rubber characteristics, e.g., resistance to oil, deformation rate (permanent compression set) under heated and pressurized conditions and rubber elasticity at high temperature. The compositions containing a hydrogenated derivative of the above block copolymer have been crosslinked to improve the above characteristics, and these crosslinked compounds are disclosed by, e.g., Japanese Patent Laid-open Publication Nos.S59-6236 and S63-57662, and Japanese Patent Publication Nos. H3-49927, H3-11291 and H6-13628.
However, the crosslinked compositions of hydrogenated block copolymers disclosed by the above patent publications still have problems of insufficient permanent compression set at high temperature, in particular at about 100° C., and tend to lose mechanical strength. Therefore, they are still short of the performance level which has been required in applications for vulcanized rubber. Moreover, they involve many moldability-related problems, e.g., low melt tension of the extruded article at high temperature thereby deteriorating its ability of keeping its shape, and extended molding cycles for injection-molded articles.
Each of the compositions disclosed by the above patent publications has a Shore A hardness of 40 or more, and is softened by increasing softening agent content. However, this causes practical problems, e.g., sticky surfaces of the molded article and bleed-out of the softening agent in the presence of stress under heating.
It is an object of the present invention to provide thermoplastic elastomer compositions excellent in softness, permanent compression set (both short- and long-term), and having controlled bleed-out of the softening agent. It is another object of the present invention to provide formed articles and composite formed articles of the above thermoplastic elastomer compositions.